Cabletron Systems manual Design Considerations For Fddi Networks, BRIM-F6 User’s Guide A-15

Design Considerations For FDDI Networks

To translate fiber path to ring length, remember that FDDI networks contain two counter-rotating rings. Under normal conditions (no failed segment), the ring length remains the same as the fiber path length. However, if a wrap occurs, the length of the fiber path length could nearly double. To safely establish the maximum ring length, divide the fiber length by two. This yields a maximum ring length 100 km (one-half of the 200 km fiber path length).

In designing your network, you must add all of the lengths of the fiber optic cables in your network to determine the total ring length. This includes main ring cables and branch cables that reach from concentrators to SASs.

A.7.2 Drive Distance

Drive distance is the limit of reliable signal propagation around the ring. (i.e., the greatest distance that a signal can travel on the ring and still be reliably received). For FDDI networks using multimode fiber the maximum drive distance is 2 km. For networks using single mode fiber, the maximum drive distance is up to 40 km (25 km typical) depending on transceiver type.

To the network designer, this means that the maximum cable length between any two network nodes must not exceed the 2 km maximum distance limit for multimode or 25 km typical distance for single mode.

In some multimode applications, existing 50/125 ∝m or 100/140 ∝m fiber operates over longer distances. However, the cable must conform to the FDDI standard for bandwidth and attenuation to remain compliant with the FDDI standard. The 40 km maximum for single mode fiber is for absolute best-case (i.e., not for practical application).

A.7.3 Attenuation

The maximum attenuation (attenuation budget) between any two active connections to the ring, according to the FDDI standard, is 11 dB. This budget includes the attenuation of the cabling, splices, connections, and optical bypass switches.

BRIM-F6 User’s Guide Page A-15